1. Field of the Invention
This invention relates to an automatic focusing device in which a sensor is arranged to receive the image of an object to be photographed; the degree and the direction of deviation from an in-focus point position, i.e. a focusing degree focusing error, is computed on the basis of an image signal produced from the sensor; and automatic focusing is accomplished by driving a lens to an in-focus position according to the result of the computation.
2. Description of the Prior Art
An ideal automatic focusing device should be capable of computing a focusing error under any condition on the basis of an image signal obtained from the sensor. In actuality, however, it becomes impossible to compute a focusing error where the error is excessive, where the contrast of an object to be photographed is low (hereinafter called a low contrast) or where the brightness of the object is low (hereinafter called a low brightness). To solve this problem, a proposal has been made for a mechanism to shift the lens a predetermined extent in the event that computation of focusing error is impossible, then to try computation with the lens at the position to which it has been thus shifted; and, if the computation is still impossible there, the lens is again shifted the predetermined extent. In this manner, the lens is repeatedly shifted a predetermined amount until it reaches a lens position whose focusing error is computable. However, where the focusing error computation is impossible due to low brightness, it is fundamentally unfeasable to obtain the focusing error even with the lens position repeatedly shifted to a predetermined extent in the above-stated manner. In that case, the effect of the searching mechanism of the prior art device has been unreliable and often resulted in waste of time and electrical energy.